Watt-hour meters, most commonly used as electric energy billing meters, are simple induction electro-mechanical meters. These meters have proven to have a high degree of accuracy and reliability through many years of service under varying ambient operating conditions. The reliability of these meters is exemplified by the fact that accrediting authorities in countries such as Canada require only sample testing of meters after which the meters can be sealed for a period of twelve years. This seal can be indefinitely extended for periods of up to eight years, depending upon the accuracy criteria of the testing.
The watt-hour meter includes a motor, a magnetic brake and a register. The torque of the motor is proportional to the power flowing through it. The magnetic brake is used to retard the speed of the motor in such a way that the rotation speed is proportional to the power flowing through it (by making the braking effect proportional to the speed of the rotor). The register is used to count the number of revolutions the motor makes. The rotor portion of the meter motor is an electrical conductor in the form of a disc which is placed between the pole faces of the stator. Magnetic fluxes induce emf's in the disc which cause eddy currents that react with the alternating magnetic field, causing torque on the disc. Since the disc is free to turn, the torque causes it to rotate. Since the torque tends to cause constant acceleration, brake magnets are placed around the disc. The strength of the magnet is chosen so that the retarding torque will balance the driving torque at a given speed.
In the existing art, the basic parts of the meter are assembled on a frame, mounted on a base, and then covered with a glass cover. The purpose of the frame is to hold in fixed relation the current stator, potential stator, disc, brake magnets and the register.
In order to support the spindle (shaft) on which the rotor is mounted, bearings that produce a minimum amount of friction are used. In existing meters there is an upper bearing and a lower bearing each slidably mounted in a bore on the frame; each slidable bearing is held by a set screw. This construction is clearly illustrated, for example, in FIG. 2 of U.S. Pat. No. 4,881,070. Each bearing is mounted a flexible pintle (guide pin) of a length which is non-resonant to any vibration frequencies encountered from no-load to maximum load. The upper and lower ends of the disc spindle are centered by graphite or polyamide rings (pilot bearings) that are inserted into recesses in respective opposite ends of the shaft. These pilot bearings function as the bearing surface for the guide pivots.
The spindle assembly is supported by the mutual repulsion of two magnets one being attached to the shaft for rotation therewith and the other mounted in the lower bearing assembly. These are referred to as upper and lower magnets. There is a temperature compensator associated with the lower magnet. The only bearing pressures in this type of rotor support are slight side thrusts on the pintles since the shaft generally does not otherwise touch either the top or bottom supports.
In the existing prior art meters the disc is centered between the brake magnets and stators by vertically moving the upper and lower bearings and using set screws to lock the bearings in position. It is both difficult and awkward to hold the bearing in the desired location while at the same time tightening its set screw, and it is furthermore difficult to gauge the distance that the bearing and hence the disc has travelled in each adjustment. The adjustment thus becomes a tedious trial and error task with no predictability to repetitive adjustments.